Cellular and colloidal separation using optical forces.

The separation or sorting of cellular and colloidal particles is currently a central topics of research. In this chapter, we give an overview of the range of optical methods for cell sorting. We begin with an overview of fluorescence and magnetically activated cell sorting. We progress to describing methods at the microfluidic scale level particularly those exploiting optical forces. We distinguish between what we term passive and active schemes for sorting. Optical forces pertinent to the sorting schemes are described, notably the gradient force and the optical radiation pressure (or scattering force). We discuss some of the most recent advances. This includes techniques without fluid flow where we have either stationary or moving light patterns to initiate separation. Further methods have shown how using an externally driven flow either counter-propagating against a light field (optical chromatography) or over a periodic light pattern (an optical potential energy landscape) may result in the selection of particles and cells based on physical attributes such as size and refractive index. We contrast these schemes with the field of dielectrophoresis where electric field gradients may separate cells and also briefly mention the upcoming area of light-induced dielectrophoresis which marries the reconfigurability of optical fields with the power of dielectrophoresis.